typedef struct _PRIVATE_CACHE_MAP_FLAGS
{
     ULONG DontUse: 16;
     ULONG ReadAheadActive: 1;
     ULONG ReadAheadEnabled: 1;
     ULONG PagePriority: 3;
     ULONG Available: 11;
} PRIVATE_CACHE_MAP_FLAGS, *PPRIVATE_CACHE_MAP_FLAGS;

Windows Vista Kernel Structures